It is known that processes exist for separating feedstreams containing molecules having differing sizes and shapes by contacting the feedstream with a molecular sieve into which one component of the feedstream to be separated is more strongly adsorbed by the molecular sieve than the other. The more strongly adsorbed component is preferentially adsorbed by the molecular sieve to provide a first product stream which is enriched in the weakly or non-adsorbed component as compared with the feedstream. After the molecular sieve is loaded to a desired extent with the adsorbed component, the conditions of the molecular sieve are varied, e.g., typically either the temperature of or the pressure upon the molecular sieve is altered, so that the adsorbed component can be desorbed, thereby producing a second product stream which is enriched in the adsorbed component as compared with the feedstream.
Important factors in such processes include the capacity of the molecular sieve for the more strongly adsorbable components and the selectivity of the molecular sieve (i.e., the ratio in which the components to be separated are adsorbed). In many such processes, zeolites are the preferred adsorbents because of their high adsorption capacity and, when chosen so that their pores are of an appropriate size, their high selectivity.
Often the zeolites used in the separation of gaseous mixtures are synthetic zeolites. Although natural zeolites are readily available at low cost, natural zeolites are often not favored as adsorbents because it has been felt that the natural zeolites are not sufficiently consistent in composition to be useful as adsorbents in such processes. However, there are relatively few synthetic zeolites with pore sizes in the range of about 3 to 4 .ANG., which is the pore size range of interest for a number of gaseous separations.
One such separation is the separation of ammonia from methane and other hydrocarbons, including ethylene and propylene, having kinetic diameters not greater than about 5 .ANG.. In the manufacture of polyethylene, ethylene-containing streams which contain ethylene, ethane and propane, together with traces (typically of the order of 10 parts per million or less) of ammonia, often must be purified to reduce the already small proportion of ammonia further before the ethylene stream reaches the polymerization reactor, because the presence of even a few parts per million of ammonia can poison commercial ethylene polymerization catalysts. The ammonia removal can be effected by passing the ethylene stream through a bed of calcium zeolite A. Although calcium A zeolite is an efficient adsorber of ammonia, it also adsorbs relatively large quantities of ethylene, and given the much greater partial pressure of ethylene in the ethylene stream, the quantity of ethylene adsorbed is much greater than that of ammonia. Thus, relatively large quantities of ethylene are wasted in the removal of the traces of ammonia. Similar problems are encountered in the propylene stream used to manufacture polypropylene.
Clinoptilolites are a well-known class of natural zeolites which have occasionally been proposed for the separation of gaseous mixtures, usually light gases such as hydrogen, nitrogen, oxygen, argon, methane, etc.
For example, European Patent Application No. 84850131.8 (Publication No. 132 239) describes a process for the separation of oxygen and argon using as the adsorbent raw clinoptilolite, i.e., clinoptilolite which has not been subjected to any ion-exchange.
The separation of gaseous mixtures of methane and nitrogen using both raw clinoptilolite and clinoptilolite which had been ion-exchanged with calcium cations is described in the following publication; T. C. Frankiewicz and R. G. Donnelly, METHANE/NITROGEN SEPARATION OVER THE ZEOLITE CLINOPTILOLITE BY SELECTIVE ADSORPTION OF NITROGEN, Chapter 11, INDUSTRIAL GAS SEPARATION, American Chemical Society, 1983. They disclose that at long adsorption times, adsorption approaches thermodynamic equilibrium and there is a tendency for adsorbed nitrogen to be replaced by methane. However, since methane diffusion is slower than nitrogen diffusion into clinoptilolite, the separation can be made on a rate basis.
Japanese Patent Application (Kokai) No. 61-255,994 discloses a process for producing a high-caloric gas comprising two adsorption zones wherein nitrogen and other non-combustible low-caloric components are removed from a feed gas, e.g., coke oven gas or methane reaction gas, which also contains hydrogen, methane and other hydrocarbons. This Japanese patent application discloses that the nitrogen is adsorbed on a clinoptilolite adsorbent that may be naturally produced clinoptilolite, natural clinoptilolite that has been crushed as required either in its original form or after ion exchange or other chemical treatment, natural clinoptilolite that has been combined with a suitable binder, then compacted and sintered, natural clinoptilolite that has merely been heat-treated, or from clinoptilolite obtained by a synthetic process. This Japanese patent application does not, however, disclose any specific cations that would be suitable as ion-exchange agents in clinoptilolite for adsorbing ammonia.
Japanese Patent Application (Kokai) No. 62-132,542 discloses an adsorbing and separating composition composed of a clinoptilolite type zeolite containing calcium cations in a mole ratio of CaO/Al.sub.2 O.sub.3 of 0.4 to 0.75. The application discloses that the composition is useful for separating molecules with a kinetic diameter of less than 3.7 .ANG. from molecules with that of 3.7 .ANG.s or greater, e.g., removal of a small quantity of nitrogen from methane gas, or bulk separation of nitrogen from a methane-containing coke oven gas or coal mine draught gas, etc. There is no specific disclosure or suggestion that this adsorbent would be suitable for separating ammonia from feedstreams containing hydrocarbons.
Accordingly, processes are sought which can separate ammonia from hydrocarbons having kinetic diameters of less than about 5 .ANG., e.g., methane, ethane, ethylene, propane and propylene, by adsorption using modified clinoptilolite adsorbents. Moreover, processes for the production of the modified clinoptilolite adsorbents are sought.